Neutron generator

ABSTRACT

A neutron generator, comprising: 
     (i) an ion source comprising an anode and a thermionic cathode disposed in an ionizable gas environment (e.g. hydrogen isotope); 
     (ii) means for heating the cathode so that the latter emits electrons which, when colliding with the gas atoms, generate ions; 
     (iii) a target; 
     (iv) an electrical gap to accelerate ions from the ion source towards the target upon impingement of the ions; and 
     (v) control means for applying voltages to the anode, cathode and electrical gap. 
     The cathode is of the dispenser type or volume type, and preferably comprises one block of material comprised of a substrate impregnated with an electron emitting material.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to neutron generating systems and moreparticularly pertains to a new and improved neutron generator especiallyadapted to traverse the narrow confines of a well or borehole, althoughuseful in a variety of other applications. Since a neutron generatorembodying the invention is ideally suited to the needs of well loggingservices, it will be described in that connection.

2. The Related Art

The use of a generator of high energy neutrons has been known for a longtime for neutron-gamma ray or neutron-neutron logging. A neutrongenerator has advantages compared with chemical neutron sources, inparticular it features a negligible amount of radiation other than thedesired neutrons; a high yield of neutrons; a controllable yield ofneutrons in bursts or continuously; neutrons at higher energies thanformerly possible; mono-energetic neutrons; and control of the generatorso as to permit its deactivation prior to withdrawal from or insertionin a well. The first five of these attributes are important in obtainingmore informative logs, while the last is valuable in minimizing healthhazards to operating personnel.

Neutron generators used in oil well logging tools usually requirecontrolled low pressure atmospheres and high intensity magnetic fields.Accordingly, for illustrative purposes, the invention is described inmore complete detail in connection with a neutron generator suitable foruse in a well logging tool.

Neutron generators usually have three major features:

(i) a gas source to supply the reacting substances, such as deuterium(H²) and tritium (H³);

(ii) an ion source comprising usually at least one cathode and an anode;electrons are emitted from the cathode surface when an electricalimpulse is applied to the anode; impact of the primary electrons on thegas molecules result in subsequent secondary electrons being strippedfrom the gas molecules, thus generating positively charged ions; and

(iii) an accelerating gap which impels the ions to a target with suchenergy that the bombarding ions collide with deuterium or tritium targetnuclei in neutron (n) generating reactions:

    H.sup.2 +H.sup.2 →He.sup.3 +n+3.26 MeV

    H.sup.2 +H.sup.3 →He.sup.4 +n+17.6 MeV

    H.sup.3 +H.sup.3 →He.sup.4 +2n+13 MeV

where He³ and He⁴ are helium isotopes, and the energy is expressed inmillions of electron volts (MeV).

Ordinarily, negative electrons and positively charged ions are producedthrough electron and uncharged gas molecule collisions within the ionsource. Electrodes of different potential contribute to ion productionby accelerating electrons to energy higher than the ionizationthreshold. Collisions of those energetic electrons with gas moleculesproduce additional ions and electrons. At the same time, some electronsand ions are lost to the anode and cathode. In this manner, the positiveand negative charges inside the ion source approach an equilibrium.Collision efficiency can be increased by lengthening the distance thatthe electrons travel within the ion source before they are neutralizedby striking a positive electrode. One known path lengthening techniqueestablishes a magnetic field which is perpendicular to theaforementioned electric field. The combined magnetic and electricalfields cause the electrons to describe a helical path within the ionsource which substantially increases the distance traveled by theelectrons within the ion source and thus enhances the collisionefficiency of the device.

This type of ion source, called "Penning ion source", is part of afamily of "cold cathode ion sources" and has been known as early as1937; see for example the article by F. M. Penning and J. H. A. Moubisin Physica 4 (1937) 1190. Examples of neutron generators including such"cold cathode ion source" used in logging tools are described e.g. inU.S. Pat. No. 3,546,512 or 3,756,682 both assigned to SchlumbergerTechnology Corporation.

However, neutron generators using Penning ion sources used in loggingtools suffer from several drawbacks.

First, the anode being at a high potential, in the range of 1 to 3 kV,the cathode suffers erosion due to energetic ion bombardment. Materialsputtered from the cathode may coat the insulator surfaces provided forelectrical insulation either of the anode or of the target. This maycause instability which is prejudicial to the operation of the ionsource. Also, this instability occurring in a space where high voltagesare involved can be detrimental to safety.

Second, most logging nuclear measurements are carried out by emittingpulses of neutrons which irradiate the earth formations, and bydetecting the radiation (neutrons or gamma rays) resulting from theinteraction of earth formation atoms and the emitted neutrons. Thus, itis critical to have a good knowledge of the characteristics of theneutron pulse, such as the neutron output (number of neutrons emitted)and the pulse timing. Such knowledge means having control over thesecharacteristics. It is highly desirable to generate neutron pulseshaving a substantially square shape, in particular a short rise time (toreach the plateau value) and a short fall time (once the voltages areturned off). However, in a Penning source, such tasks are difficultbecause the charge populations in the source, particularly the electronpopulation, do not reach equilibrium instantaneously; see F. Chen, J.Appl. Phys. 56 (11) 3191, 1984. The rate at which the charge populationsapproach the equilibrium depends strongly on the gas pressure in thesource. This effect manifests itself in the slow rise time of theneutron pulse, and a delay, typically a few microseconds (althoughsometimes variable with operation conditions), between the time thevoltage appears at the anode and the start of the neutron pulse. Sincethe cathode and anode surface conditions are not identical betweendifferent neutron tubes, different pressures are often required toachieve the same neutron output. This makes the timing control of thesource all the more difficult that it is essentially a function of theparticular neutron generator, and may vary over the operating lifetimeof the neutron generator.

Third, the high voltage required for a Penning ion source (1-3 kV) isgenerally produced via a pulse transformer. The transformers aredesigned for a certain pulse width. Thus, changing pulse length resultsin altering the performance, most noticeably, the neutron pulse shape.There have been some attempts to improve the neutron pulse shapegenerated from a cold cathode ion source. In particular, the article"Neutron Generators for Wireline Application," from R. Ethridge et al.,1990 IEEE Nuclear Science Symposium Conference Record, Arlington, Va.,Oct. 22-23, 1990, Vol. 2 of 2, describes a cold cathode source whereinthe pulse transformer is provided with a "clamping" circuit designed todecrease the fall time of the pulse. However, such clamping circuits:(i) do not seem to improve the rise time of the neutron pulse; (ii)require additional power; (iii) and increase the overall size of thecontrol circuit.

Fourth, the known cold cathode sources can usually operate in any one ofseveral discharge modes according to the relative ion and electronpopulations and different plasma sheath structures. The anode voltage,the magnetic field and the gas pressure determine the operating point atwhich the production and loss of electrons and ions are at balance. Inaddition, under certain conditions, the operating point is unstable nearcertain mode boundaries. The transition from one mode to the other canlead to a substantial change in the ion beam density and electronextraction efficiency, and, with control circuits currently used thatregulate the beam current by lowering the gas pressure, to a reductionin gas pressure that can result in oscillations about the mode boundary.The resulting neutron output variations are detrimental to the overallquality of the measurements.

Fifth, the means for generating the magnetic field, intended to lengthenthe electrons path, are relatively cumbersome and increase the overalldimensions and weight of the neutron generator. This is of concern in alogging tool where room is limited.

An alternative to the cold cathode ion sources are "hot cathode" ionsources, proposed as early as 1939, associated to a spectrograph, asdepicted e.g. in the article "Focused Beam Source of Hydrogen and HeliumIons" by G. W. Scott Jr., in Physical Review, May 15, 1939, volume 55.Further developments in the same technical area provided somemodifications to the basic hot cathode ion source; see e.g. the article"An Electrostatically Focused Ion Source and its Use in a Sealed-OffD.C. Neutron Source" by J. D. L. H. Wood and A. G. Crocker, NuclearInstruments and Methods, 21 (1963) pages 47-48; or the article "ElectronBombardment Ion Source for Low Energy Beams" by S. Dworetsky et al., inThe Review of Scientific Instruments, November 1968, volume 39, No. 11.A "hot cathode" typically comprises a material susceptible, when heated,to emit electrons. The cathode is disposed above, or concentrically to,the anode. An extracting electrode (also called focusing electrode) isplaced at the front of the anode to extract ions, generated fromcollisions between electrons and gas molecules, and focus such ions soas to form an ion beam.

Hot cathode ion sources by themselves bring some improvements withrespect to cold cathode ion sources. Hot cathode sources for instance:(i) do not always require a magnetic field, and this allows asubstantial reduction in weight and dimensions; (ii) are able togenerate an optimum electron flux in a relatively short period of timeafter the voltage pulse is applied to the anode; (iii) as being used insealed neutron generator, do not show troublesome mode transitions inthe range of gas pressure where these devices normally operate; and (iv)do not require a high anode or cathode voltage when used in neutrongenerators including a discharging gas made of deuterium and tritium;this reduced voltage entails a reduction in electrode erosion.

However, hot cathode ion sources present drawbacks of their own comparedto cold cathode ion sources, such as: (i) additional power; (ii) arelatively reduced lifetime at least for most of hot cathode filamentmaterials, and (iii) the need for a specific structure to support thehot cathode and anode, especially in view of the severe shock andvibration conditions encountered during logging operations.

Moreover, according to applicant's knowledge, the known hot cathode ionsources were implemented in laboratories and designed mainly forexperimental purpose, which applications are not subjected to the severeenvironmental constraints typical of the logging techniques. In otherwords, performances of these known hot cathode ion source could beconsidered sufficient for laboratory measurements but would not beacceptable for logging applications, even assuming they could bedirectly implemented in a logging tool. Among others, one could mention,as constraints specific to logging applications: weight and dimensions,safety, neutron pulse shape, neutron output, power requirements, andoperating lifetime.

Accordingly, although the neutron generators used so far in the loggingtechniques have been working relatively satisfactory, there still is aneed for an improvement to the neutron output and especially to theneutron pulse shape.

SUMMARY OF THE INVENTION

It is a general object of the present invention to provide an improvedneutron generator, especially suitable for logging techniques.

It is a first particular object of the invention to propose a neutrongenerator providing neutron pulses having a substantially square shape,to wit: (i) a sharp rise; (ii) a substantially "flat" plateau; and (iii)a sharp cut-off (very abrupt termination of the neutron burst).

A second specific object of the invention is to propose a neutrongenerator of reduced weight and dimensions.

It is a third particular object of the invention to provide a neutrongenerator with relatively low ion energies at the low source aperture,thus reducing metal erosion of the electrodes and improving the voltagestability.

It is a fourth specific object of the invention to propose a neutrongenerator wherein the rise edge (or leading edge) of the neutron fluxappears with a reduced delay after the voltage pulse is applied to theanode, thus simplifying the timing control process of the pulses.

It is a fifth specific object of the invention to provide a neutrongenerator with a hot cathode ion source requiring minimum power tooperate.

It is a sixth object of the invention to provide a neutron generatorrequiring pulses of relatively low voltages to produce the ion beam,making it possible to eliminate the need for an ion source transformer.(by using, e.g. solid state switches).

It is a seventh object of the invention to provide a neutron generatorwherein pulses of very different duration can be applied to the ionsource, allowing for complex series of pulse lengths during a welllogging measurement without requiring a reconfiguration of the ionsource pulsing circuit.

It is an eighth specific object of the invention to provide a neutrongenerator showing a high mechanical or electrical ruggedness, especiallyfor use in a logging tool.

These objects and other are attained, according to the invention, with alogging tool for investigating earth formations surrounding a borehole,comprising:

1) a sonde incorporating at least one radiation detector; and

2) a neutron generator comprising:

(i) an ion source comprising an anode and a thermionic cathode disposedin an ionizable gas environment;

(ii) means for heating the cathode so that the latter emits electronswhich, when colliding with the gas atoms, generate ions;

(iii) a target;

(iv) an electrical gap to accelerate ions from the ion source towardsthe target upon impingement of the ions; and

(v) control means for applying voltages to the anode, cathode andelectrical gap.

The cathode is preferably of the dispenser or volume type. The terms"thermionic", "dispenser" and "volume" will be hereafter explained.

In a preferred embodiment, the gas comprises at least one hydrogenisotope and the gas environment constitutes a sealed chamber.

The cathode advantageously includes a substratum made of porous tungstenand an emitter material including barium oxide and/or strontium oxide.

The voltage supply means for the cathode are distinct from the cathodeheating means.

The neutron generator may further comprise means for preventing slowions still present in the ion source at the end of the voltage pulse,from leaving the ion source. The preventing means comprises a cut-offelectrode disposed at the end of the ion source and which is submittedto voltage pulses synchronized with and complementary to pulses appliedto the anode, and to a positive voltage between the pulses. The cut-offelectrode includes a convex mesh screen.

The invention also relates to a neutron generator comprising:

an ion source comprising an anode and a dispenser or volume type cathodedisposed in an ionizable gas environment including at least one hydrogenisotope;

means for heating the cathode so that the latter emits electrons which,when colliding with the gas atoms, generate ions;

a target;

an electrical gap to accelerate ions from the ion source towards thetarget upon impingement of the ions; and

control means for applying voltages to the anode, cathode and electricalgap.

The characteristics and advantages of the invention will appear betterfrom the description to follow, given by way of a non limiting example,with reference to the appended drawing in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section view of a neutron generator according to theinvention;

FIGS. 2A and 2B are schematic representations of respective alternateembodiments of the cathode; and

FIG. 3 is an example of plot of neutron output versus time, showing thecorresponding neutron pulse.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a neutron generator 10 which may be used in a logging toolsuch as described e.g. in U.S. Pat. Nos. 4,794,792, 4,721,853 or4,600,838, which are herein incorporated by reference. The majorcomponents of the neutron generator 10 are a hollow cylindrical tube 11made of an insulating material such as alumina ceramic and having itsrespective longitudinal extremities fixed to a ceramic ring 12 and aconductive ring 13, an ion source 45, a gas supply means 25, anextracting electrode 50, and a massive copper target electrode 15. Atransverse header 14 and the target electrode 15 close the rings 12 and13, respectively, to provide a gas-tight cylindrical envelope. Ring 12comprises parallel transversely disposed flanges 6, 7, 8, and 9,providing electrically conductive paths and sturdy support for thegenerator components as described subsequently in more complete detail.Flanges 6-9 are substantially equally spaced along ring 12, betweenheader 14 and the corresponding extremity of tube 11. The gas supplymeans 25 is disposed transversely to the longitudinal axis I--I of thegenerator 10, between first flange 6 and second flange 7, closest toheader 14. The gas supply means 25 comprises a helically wound filament26 of tungsten, which may be heated to a predetermined temperature by anelectric current from a gas supply power means 105 to which both ends26a and 26b of filament 26 are connected.

A film 44 of zirconium or the like, for absorbing and emitting deuteriumand tritium, is coated on the intermediate turns of the filament 26 inorder to provide a supply of these gases and to control gas pressureduring generator operation. Due to physical isolation, a substantiallyuniform temperature can be maintained along the coated intermediateturns of the filament helix 26.

As the gases released from the film 44 are withdrawn from the atmospherewithin the envelope for ion generation, more gases are emitted torestore the envelope gas pressure to a level commensurate with thetemperature of the intermediate portion of the filament helix 26. Thegases emitted by the film 44 diffuse through holes provided in flanges7-9, i.e. a hole 31 in second flange 7, a hole 33 in third flange 8 andholes 34, 35 in fourth flange 9. The gases emitted finally enter an ionsource 45 interposed between the gas supply means 25 and the extremityof tube 11 facing ring 12. An annular shaped electrical insulator 90 isinterposed between tube 11 and ring 12. More details on the structure ofthe neutron generator can be found e.g. in U.S. Pat. Nos. 3,756,682; or3,775,216; or 3,546,512, which are herein incorporated by reference.

The ion source 45 comprises a cylindrical hollow anode 57 aligned withthe longitudinal axis I--I of the generator 10 and made out of either amesh or a coil. Typically, a positive ionizing potential (either director pulsed current) comprised in the range of 100-300 volts relative tothe cathode, is applied to the anode 57. In one exemplary embodiment ofthe invention, the anode 57 is about 0.75 inch (1.9 cm) long and has adiameter of approximately 0.45 inch (1.14 cm). The anode 57 is securedrigidly to flange 9, e.g. by conductive pads 60.

The ion source 45 also includes a cathode 80 disposed close to theoutside wall of the anode 57, in a substantially median position withrespect to the anode. The cathode 80 comprises an electron emitter 81consisting of a block of material susceptible, when heated, to emitelectrons. Emitter 81 is fixed (e.g. by brazing) to the U-shaped end 82of an arm 84 being itself secured to flange 8. The arm 84 provides alsoan electrical connection between the emitter 81 and a hot cathode heatercurrent means 100 able to generate e.g. a few watts for heating theemitter. Heater current 100 is known per se (see U.S. Pat. Nos.3,756,682, 3,775,216 or 3,546,512) and thus does not need to be furtherdescribed. According to an alternate embodiment shown on FIG. 2A, thecathode 80 could also comprise two arms (similar to arm 84), eachprovided at one of its ends with a block of dispenser material, botharms being disposed outside the hollow anode 57. This embodiment(cathode disposed outside the anode) prevents the material evaporatedfrom the cathode from coating the surface of suppressor 75 causingenhanced field emission.

In a further alternate embodiment shown on FIG. 2B, the cathode 80 mayalso comprise a single arm provided at one end with an emitter, the armbeing disposed inside the hollow anode 57, substantially in the centerthereof. According to this embodiment, the cathode emitting surfaces areso arranged that electron emission is perpendicular to the axis of theion source. This embodiment reduces the amount of cathode material beingdeposited on the suppressor surface.

Now described in more detail is the structure of the cathode 80. Thethermionic cathode 80 comprises an emitter block including a materialforming a substratum and a material susceptible to emit electrons.Thermionic cathodes here mean heated cathodes, as opposed to coldcathodes which emit electrons when not heated. The thermionic cathodescan be broken down into: (i) those with inherent electron emissioncapability if they can be heated high enough in temperature withoutmelting (e.g. pure tungsten or tantalum or lanthanum hexa boride), and(ii) those to which use a low work function material is applied, eitherto the surface of a heated substratum (such as thoria coated tungsten,oxide coated) [called "oxide cathode"], or impregnated by bulk into aporous substrate [called "dispenser" cathode]. General information onthermionic cathodes can be found in the book "Materials and Techniquesfor Electron Tubes" by W. Kohl, Reinhold Publishing, 1960, pages519-566, which is herein incorporated by reference. In other words,"oxide" cathode involve what could be called a "surface" reaction,whereas in a "dispenser" cathode there occurs what could be called a"volume" reaction. General information on "dispenser" or "volume" typecathodes can be found e.g. in the article "Surface Studies of Barium andBarium Oxide on Tungsten and its Application to Understanding theMechanism of Operation of an Impregnated Tungsten Cathode" by R. Forman,in Journal of Applied Physics, vol. 47, No 12, December 1976, pages5272-5279; or in the article "A Cavity Reservoir Dispenser Cathode forCRT's and Low-cost Traveling-wave Tube Applications" by L. R. Falce, inIEEE transactions on electron devices, vol 36, No 1, January 1989.Cathodes of the "oxide" or "surface" type are described in the article"Compact Pulsed Generator of Fast Neutrons" by P. O. Hawkins and R. W.Sutton, The Review of Scientific Instruments, March 1960, Vol. 31,Number 3, Pages 241-248; in "Focused Beam Source of Hydrogen and HeliumIons" by G. W. Scott, Jr., in Physical Review, May 15, 1939, vol 55,pages 954-959; in U.S. Pat. No. 3,490,944 or U.S. Pat. No. 3,276,974; orin the article "Operation of Coated Tungsten Based Dispenser Cathodes inNonideal Vacuum" by C. R. K. Marrian and A. Shih, in IEEE Transactionson Electron Devices, vol. 36, No 1, January 1989. All of the abovementioned documents are incorporated herein by reference.

The thermionic cathode 80 of the ion source of the present invention ispreferably of the "dispenser" or "volume" type. A dispenser cathode usedin a hydrogen environment maximizes electron emissions per heater powerunit compared to other thermionic type cathodes (such as LaB₆ or W),while operating at a moderate temperature. The emitter block 81comprises a substrate made of porous tungsten, impregnated with amaterial susceptible to emit electrons, such as compounds made withcombinations of e.g. barium oxide and strontium oxide. Each cathode hasdifferent susceptibility to their operating environment (gas pressureand gas species). Dispenser cathodes are known to be the most demandingin terms of the vacuum requirements and care that is needed to avoidcontamination. One, among others, of the (novel and non-obvious)features of the invention includes using, in a neutron generator, adispenser cathode which works as long as several hundred hours in ahydrogen gas environment of pressure on the order of several mTorr,providing an average electron emission current of from 50 to 80 mA yetrequiring only a few watts of heater power.

The cathode 80 according to the invention is provided with hot cathodeheater current 100 which is distinct from the ion source voltage supply102. Such implementation permits a better control of both heater currentmeans 100 and voltage supply 102.

The extracting electrode 50 is disposed at the end of the ion source 45facing target electrode 15, at the level of the junction between tube 11and ring 12. The extracting electrode 50 is supported in fixed relationto the ring 12 by a fifth flange 32. The extracting electrode 50comprises a massive annular body 46, e.g. made of nickel or an alloyedmetal such as KOVAR (trademark), and which is in alignment with thelongitudinal axis I--I of the tube 11. A central aperture 47 in the body46 diverges outwardly in a direction away from the ion source 45 toproduce at the end of body 46 facing target electrode 15 a torus-shapedcontour 51. The smooth shape contour 51 reduces a tendency to voltagebreakdown that is caused by high electrical field gradients.

Moreover, the extracting electrode 50 provides one of the electrodes foran accelerating gap 72 that impels ionized deuterium and tritiumparticles from the source 45 toward a deuterium- and tritium-filledtarget 73. The target 73 comprises a thin film of titanium or scandiumdeposited on the surface of the transverse side, facing ion source 45,of the target electrode 15.

The potential that accelerates the ions to the target 73 is established,to a large extent, between the extracting electrode 50 and a suppressorelectrode 75 hereafter described. The suppressor electrode 75 is aconcave member that is oriented toward the target electrode 15 and has acentrally disposed aperture 78 which enables the accelerated ions tofrom the gap 72 to the target 73. The aperture 78 is disposed betweenthe target 73 and the extracting electrode 50. The suppressor electrode75 is connected to a high voltage supply means 103 which is alsoconnected, through a resistor "R" to the ground. In order to preventelectrons from being extracted from the target 73 upon ion bombardment(these extracted electrons being called "secondary electrons"), thesuppressor electrode 75 is at a negative voltage with respect to thevoltage of the target electrode 15.

The velocity of the ions leaving the ion source 45 is, on an average,relatively lower than ion velocity in a known Penning source.Consequently, these slow moving ions tend to generate a tail in theneutron pulse, at the moment the voltage pulse is turned off. Thepresence of an end tail is detrimental to the pulse shape which, asalready stated, is of importance. The present invention remedies thissituation by adding to the extractor a cut-off electrode, in the form ofa mesh screen 95, which is fixed, e.g. by welding, to the aperture 47 ofthe extracting electrode 50, facing the ion source 45. The mesh screen95 (cut-off electrode) is e.g. made of high transparency molybdenum. Thecut-off electrode 95 is submitted to voltage pulses synchronized withand complementary to the voltage pulses applied to the anode 57. Thepulses applied to cut-off electrode 95 are positive and e.g. of 100 to300 volts. In an alternate embodiment, the cut-off electrode 95, insteadof being submitted to voltage pulses, is maintained at a positivevoltage, of e.g. a few volts. This low positive voltage prevents theslow ions produced at the end of the pulse in the ion beam from leavingthe ion source, and thus allows one to truncate the terminal part of theion beam, which in turn provides a sharp cut-off at the end of theneutron pulse (i.e. a short fall time). The cut-off electrode 95 ispreferably made of a metallic grid in the form of a truncated sphere,and its concavity turned towards the target 73. Part of the mesh screen95 might protrude inside cylindrical hollow cathode 57. FIG. 3 shows twoexamples of neutron pulses obtained respectively with cut-off electrode(solid line) and without cut-off voltage (dotted line), everything elsebeing equal. The benefit to the neutron pulse shape (especially the falltime) derived from the cut-off electrode is easily appreciated from FIG.3.

In an alternate embodiment, (wherein the extractor 50 is not providedwith the cut-off screen 95), the end tail of the ion beam is truncatedby applying a positive voltage pulse to the extracting electrode 50.

In order to generate a controlled output of neutrons, continuously or inrecurrent bursts, an ion source voltage supply means 102 provides powerfor the bombarding ion beam. For pulse operation, an ion source pulser101 is provided at the output of ion source voltage supply 102 toregulate the operation of voltage supply to the ion source. The ionsource pulser 101 has a direct output connected to the anode 57 (viaflange 9) and a complementary output connected to extracting electrode50. The high voltage supply 103, the ion source voltage supply 102, andthe ion source pulser 101 may be of any suitable type such as e.g.described in U.S. Pat. Nos. 3,756,682 or 3,546,512, already referred to.A gas supply means regulator 104 (connected to the high voltage supplymeans 103) regulates, through a gas supply power means 105, theintensity of the ion beam by controlling the gas pressure in theenvelope. The current flowing through resistor r provides a measure ofion beam current which enables the gas supply regulator 104 to adjustthe generator gas pressure accordingly. The voltage developed by thehigh voltage supply 103, moreover, is applied directly to the suppressorelectrode 75 and through a resistor R to the target electrode 15. Thevoltages thus developed provide the accelerating and suppressorvoltages, respectively. During operation, current is passed through thefilament 26 of the gas supply 25 in an amount regulated by the gassupply regulator means 104 to achieve a deuterium-tritium pressurewithin the generator envelope that is adequate to obtain a desired ionbeam current and ad hoc conditions for the generator to operate.

The high voltage established between the extracting electrode 50 and thesuppressor electrode 75 produces a steep voltage gradient thataccelerates deuterium and tritium ions from the electrode aperture 47 inextracting electrode 50 toward the target 73. The energy imparted to theions is sufficient to initiate neutron generating reactions between thebombarding ions and the target nuclei and to replenish the target 73with fresh target material. Initial bombardment of a fresh target 73 by,for example, a half-and-half mixture of deuterium and tritium ions,produces relatively few neutrons. As increasing quantities of impingingions penetrate and are held in the lattice of the target, however, theprobability for nuclear reactions increases. Thus, after a short periodof ion bombardment, a continuous or pulsed output ranging from 10⁷ to10⁹ neutrons per second is reached.

As previously described, the regulator 104 regulates the power suppliedto the filament 26 and thereby manipulates the tube gas pressure and theion beam intensity to produce the desired neutron output. If the neutronoutput should increase as a result of an increase in the current, acorresponding increase in current through the resistor causes theregulator 104 to decrease the filament power supply and thereby reducethe gas pressure within the generator. The lower gas pressure in effectdecreases the number of ions available for acceleration, and thusrestores the neutron output to a stable, predetermined value. Similarly,a decrease in the current through the resistance causes the regulator104 to increase the generator gas pressure.

If desired, the neutron output can be monitored directly, and either theion source voltage supply or the high voltage power supply can becontrolled automatically or manually to achieve stable generatoroperation. In the event the generator is supplied only with deuteriumgas, neutrons are produced as a result of deuterium-deuteriuminteractions, rather than the deuterium-tritium reactions considered inthe foregoing illustrative description.

The present invention provides the following advantages, as compared tothe prior art neutron generators.

Since no magnet is necessary, the neutron generator is lighter and ofsmaller dimensions than the prior art generators. This is a substantialimprovement for logging applications due to the limited space availablein the logging tools.

The use of a dispenser cathode virtually cancels, or at leastsubstantially reduces, the delay between the time the generator isturned on and the production of neutrons, and thus provides a sharp riseof neutron burst. This also results in an improved burst timing control.

Also, the thermionic cathode operates without troublesome plasma modetransitions responsible for disturbing jumps in the neutron output, andfor difficulties in using the beam control feedback loop with thereservoir heater.

The erosion of the extractor and consequent coating of insulatorsurfaces, by sputtered metal due to ion bombardment, is substantiallyreduced because of the relatively low anode voltage. The reduced anodevoltage allows one to use simplified pulsing circuitry.

The voltage applied to the cut-off screen-electrode 95 allows the tailof the ion beam to be cut-off, made mainly of slow ions, and thus allowsthe generation of a neutron pulse showing a sharp end edge.

Finally, the lifetime of the cathode is in the range of several hundredhours in a hydrogen gas environment of pressure on the order of severalmTorr providing an average electron emission current of from 50 to 80mA, yet requiring only a few watts of heater power.

Above all, the invention is beneficial in term of pulse shape. Inparticular, the neutron pulse shows the following characteristics, ascan be seen from FIG. 3:

the time required for the instantaneous neutron output to reach itsmaximum, called plateau, measured from the instant when the voltage isapplied to said cathode, is less than 1.5 microsecond;

the fall time, i.e. the period of time between the instant when thevoltage applied to said cathode is turned off and the instant when theinstantaneous neutron output falls to 10% of the plateau, is less than0.5 microsecond;

the neutron output reaches a plateau which remains constant within a 10%range thereof, over a pulse time width comprised between 5 and 500microseconds;

the time lag between the instant when the voltage is applied to saidcathode and the instant when the instantaneous neutron output reaches10% of the plateau, is less than 0.5 microsecond; another benefit isthat the time lag is independent of operational parameters of the ionsource, such as gas pressure; and

the rise time for the neutron output to reach 90% of the plateau,measured from the time when the neutron output is 10% of said plateau,is less than 1 microsecond.

What is claimed is:
 1. A neutron generator comprising:(i) an ion sourcecomprising an anode and a dispenser cathode disposed in an ionizable gasenvironment; (ii) means for heating said cathode so that the latteremits electrons which, when colliding with said gas atoms, generateions; (iii) a target; (iv) an electrical gap to accelerate ions fromsaid ion source towards said target upon impingement of said ions; and(v) control means for applying voltages to said anode, cathode andelectrical gap, wherein a voltage applied to said anode by said controlmeans is between 100 and 300 Volts to substantially reduce metalsputtering within the neutron generator.
 2. The neutron generatoraccording to claim 1, wherein said gas comprises at least one hydrogenisotope.
 3. The neutron generator according to claim 2, wherein said gasenvironment constitutes a sealed chamber.
 4. The neutron generatoraccording to claim 1, wherein said cathode comprises at least one blockof material comprised of a substrate impregnated with an electronemitting material.
 5. The neutron generator according to claim 4 whereinsaid substrate is tungsten and said emitter material includes bariumoxide.
 6. The neutron generator according to claim 1, wherein saidvoltages are in the form of square voltage pulses.
 7. The neutrongenerator according to claim 1 wherein said voltage applying means forsaid cathode is distinct from said cathode heating means.
 8. The neutrongenerator according to claim 1, wherein said anode is made of a hollowelongated body permeable to electrons.
 9. The neutron generatoraccording to claim 8, wherein said anode is made of a cylindricalmetallic coil.
 10. The neutron generator according to claim 8, whereinsaid anode is made of a cylinder-shaped mesh.
 11. The neutron generatoraccording to claim 4, wherein said block is disposed at one end of anarm connected to said heating means and to said control means.
 12. Theneutron generator according to claim 8, wherein said cathode is disposedinside said anode.
 13. The neutron generator according to claim 8,wherein said cathode is disposed outside said anode.
 14. The neutrongenerator according to claim 11 wherein said cathode comprises two armsdisposed diametrically on the outside of said anode.
 15. The neutrongenerator according to claim 1, further comprising an extractingelectrode disposed at the end of said ion source facing said target andsubmitted to a voltage complementary to the anode voltage.
 16. Theneutron generator according to claim 15, wherein the end of saidextracting electrode facing said target is torus shaped.
 17. The neutrongenerator according to claim 6, further comprising means for preventingslows ions, still present in said ion source at the end of said voltagepulse, from leaving said ion source.
 18. The neutron generator accordingto claim 17 wherein said preventing means comprises a cut-off electrodedisposed at the end of the ion source and which is submitted to voltagepulses synchronized with and complementary to pulses applied to saidanode, and to a positive voltage between said pulses.
 19. The neutrongenerator according to claim 18 wherein said cut-off electrode includesa mesh screen.
 20. The neutron generator according to claim 19 whereinsaid mesh screen is in the form of a truncated sphere having itsconcavity facing said target.
 21. The neutron generator according toclaim 17 wherein said preventing means comprises means for applying tosaid extracting electrode negative voltage pulses synchronized withpulses applied to said anode, and a positive voltage between saidpulses.
 22. The neutron generator according to claim 3 comprising acylindrical insulator disposed between said ion source and said target.23. The neutron generator according to claim 22 wherein said insulatoris made of ceramic.
 24. The neutron generator according to claim 1wherein said gas environment comprises a gas supply means incorporatinga helical filament coated with material able, when heated, to emit atomsof at least one hydrogen isotope and disposed transversely to thelongitudinal axis of the accelerating gap.
 25. The neutron generatoraccording to claim 23 wherein the gas pressure in said gas environmentis comprised between 0.5 milliTorr and 20 milliTorr.
 26. A neutrongenerator comprising:an ion source comprising an anode and a dispenseror volume type cathode disposed in an ionizable gas environmentincluding at least one hydrogen isotope; means for heating said cathodeso that the latter emits electrons which, when colliding with said gasatoms, generate ions; a target; an electrical gap to accelerate ionsfrom said ion source towards said target upon impingement of said ions;and control means for applying voltages to said anode, cathode andelectrical gap, wherein a voltage applied to said anode by said controlmeans is between 100 and 300 Volts to substantially reduce metalsputtering within the neutron generator.
 27. A logging tool forinvestigating earth formations surrounding a borehole, comprising asonde incorporating at least one radiation detector and a neutrongenerator, said neutron generator comprising:(i) an ion sourcecomprising an anode and a dispenser cathode disposed in an ionizable gasenvironment; (ii) means for heating said cathode so that the latteremits electrons which, when colliding with said gas atoms, generateions; (iii) a target; (iv) an electrical gap to accelerate ions fromsaid ion source towards said target upon impingement of said ions; and(v) control means for applying voltages to said anode, cathode andelectrical gap, wherein a voltage applied to said anode by said controlmeans is between 100 and 300 Volts to substantially reduce metalsputtering within the neutron generator.
 28. A neutron generator forlogging applications, comprising:a source of ionizable gas; an ionsource for ionizing said gas and including an anode and a dispenser typecathode designed to emit electrons able to impinge on gas atoms so as togenerate ions; a target spaced apart from said ion source by anaccelerating gap, and being able to emit neutrons upon impingement ofions issued from said ion source; control means for applying voltages tosaid anode, cathode and electrical gap; and means for operating saidcontrol means such that the rise time for the neutron output to reach90% of the maximum output plateau), measured from the time when theneutron output is 10% of said plateau, is less than 1 microsecond.
 29. Aneutron generator for logging applications, comprising:a source ofionizable gas; an ion source for ionizing said gas and including ananode and a dispenser type cathode designed to emit electrons able toimpinge on gas atoms so as to generate ions; a target spaced apart fromsaid ion source by an accelerating gap, and being able to emit neutronsupon impingement of ions issued from said ion source; control means forapplying voltages to said anode, cathode and electrical gap; and meansfor operating said control means such that the time lag between theinstant when the voltage is applied to said cathode and the instant timewhen the instantaneous neutron output reaches 10% of the maximum output(plateau), is less than 0.5 microsecond.
 30. A neutron generator forspectral logging applications, comprising:a source of ionizable gas; anion source for ionizing said gas and including an anode and a dispensertype cathode designed to emit electrons able to impinge on gas atoms soas to generate ions; a target spaced apart from said ion source by anaccelerating gap, and being able to emit neutrons upon impingement ofions issued from said ion source; control means for applying pulsingvoltages to said anode, cathode and electrical gap; and means foroperating said control means such that the neutron output reaches amaximum value (or plateau) which remains constant within a 10% rangethereof, over a pulse time width comprised between 18 and 25microsecond.
 31. A neutron generator for logging applications,comprising:a source of ionizable gas; an ion source for ionizing saidgas and including an anode and a dispenser type cathode designed to emitelectrons able to impinge on gas atoms so as to generate ions; a targetspaced apart from said ion source by an accelerating gap, and being ableto emit neutrons upon impingement of ions issued from said ion source;control means for applying voltages to said anode, cathode andelectrical gap; and means for operating said control means such that thefall time between the instant when the voltage applied to said cathodeis turned off and the instant time when the instantaneous neutron outputfalls to 10% of the maximum output (plateau), is less than 0.5microsecond.
 32. A neutron generator for spectral logging applications,comprising:a source of ionizable gas; an ion source for ionizing saidgas and including an anode and a dispenser type cathode designed to emitelectrons able to impinge on gas atoms so as to generate ions; a targetspaced apart from said ion source by an accelerating gap, and being ableto emit neutrons upon impingement of ions issued from said ion source;control means for applying voltages to said anode, cathode andelectrical gap; and means for operating said control means such that thetime required for the instantaneous neutron output to reach its maximum(plateau) value, measured from the instant time when the voltage isapplied to said cathode, is less than 1.5 microsecond.
 33. A method forinvestigating earth formation surrounding a borehole, comprising thesteps of:irradiating, at a first given location in the borehole, theborehole materials and the earth formation with bursts of neutrons froma neutron generator including an ion source comprising an anode and adispenser cathode disposed in an ionizable gas environment, by applyingvoltage pulses to the cathode and heating the dispenser cathode;detecting, at a second given location in the borehole, radiationresulting from interaction of the neutrons with the formation;generating signals representative of the radiation; controlling theneutron output during the start of the neutron burst such that the risetime for the neutron output to reach 90% of its plateau, measured fromthe time when the neutron output is 10% of the plateau, is less than 1microsecond; and determining from the signals a characteristic of theearth formation surrounding the borehole.
 34. A method for investigatingearth formation surrounding a borehole, comprising the stepsof:generating bursts of neutrons from a neutron generator including anion source comprising an anode and a dispenser cathode disposed in anionizable gas environment, by applying voltage pulses to the cathode andheating the cathode; irradiating, at a first given location in theborehole, the borehole materials and the earth formation with bursts ofneutrons; detecting, at a second given location in the borehole,radiation resulting from interaction of the neutrons with the formation;generating signals representative of the radiation; controlling theneutron burst during the start of the neutron burst such that the timelag between the instant when the voltage is applied to the cathode andthe instant time when the instantaneous neutron output reaches 10% ofits plateau, is less than 0.5 microsecond; and determining from thesignals a characteristic of the earth formation surrounding theborehole.
 35. A method for investigating earth formation surrounding aborehole, comprising the steps of:irradiating, at a first given locationin the borehole, the borehole materials and the earth formation withbursts of neutrons from a neutron generator including an ion sourcecomprising an anode and a dispenser cathode disposed in an ionizable gasenvironment, by applying voltage pulses to the cathode and heating thedispenser cathode; detecting, at a second given location in theborehole, radiation resulting from interaction of the neutrons with theformation; generating signals representative of the radiation;controlling the neutron output such that the neutron output reaches aplateau which remains constant within a 10% range thereof, over a bursttime width comprised between 18 and 25 microsecond; and determining fromthe signals a characteristic of the earth formation surrounding theborehole.
 36. A method for investigating earth formation surrounding aborehole, comprising the steps of:generating bursts of neutrons from aneutron generator including an ion source comprising an anode and adispenser cathode disposed in an ionizable gas environment, by applyingvoltage pulses to the cathode and heating the cathode; irradiating, at afirst given location in the borehole, the borehole materials and theearth formation with bursts of neutrons; detecting, at a second givenlocation in the borehole, radiation resulting from interaction of theneutrons with the formation; generating signals representative of theradiation; controlling the neutron output such that the fall timebetween the instant when the voltage applied to the cathode is turnedoff and the instant time when the instantaneous neutron output falls to10% of its plateau, is less than 0.5 microsecond; and determining fromthe signals a characteristic of the earth formation surrounding theborehole.
 37. A method for investigating earth formation surrounding aborehole, comprising the steps of:generating bursts of neutrons from aneutron generator including an ion source comprising an anode and adispenser cathode disposed in an ionizable gas environment, by applyingvoltage pulses to the cathode and heating cathode; irradiating, at afirst given location in the borehole, the borehole materials and theearth formation with bursts of neutrons; detecting, at a second givenlocation in the borehole, radiation resulting from interaction of theneutrons with the formation; generating signals representative of theradiation; controlling the neutron output during the neutron burst suchthat the time required for the instantaneous neutron output to reach aplateau, measured from the instant time when the voltage is applied tothe cathode, is less than 1.5 microsecond; and determining from thesignals a characteristic of the earth formation surrounding theborehole.
 38. A method for investigating earth formation surrounding aborehole, comprising the steps of:generating bursts of neutrons from aneutron generator including an ion source comprising an anode and adispenser cathode disposed in an ionizable gas environment, by applyingvoltage pulses to the cathode and heating the cathode; irradiating, at afirst given location in the borehole, the borehole materials and theearth formation with bursts of neutrons; detecting, at a second givenlocation in the borehole, radiation resulting from interaction of theneutrons with the formation; generating signals representative of theradiation; controlling the neutron output such that: (i) the rise timefor the neutron output to reach 90% of its plateau, measured from thetime when the neutron output is 10% of the plateau, is less than 1microsecond; (ii) the fall time between the instant when the voltageapplied to the cathode is turned off and the instant time when theinstantaneous neutron output falls to 10% of the plateau, is less than0.5 microsecond; and determining from the signals a characteristic ofthe earth formation surrounding the borehole.
 39. A method forinvestigating earth formation surrounding a borehole, comprising thesteps of:generating bursts of neutrons from a neutron generatorincluding an ion source comprising an anode and a dispenser cathodedisposed in an ionizable gas environment, by applying voltage pulses tothe cathode and heating the cathode; irradiating, at a first givenlocation in the borehole, the borehole materials and the earth formationwith bursts of neutrons; detecting, at a second given location in theborehole, radiation resulting from interaction of the neutrons with theformation; generating signals representative of the radiation;controlling the neutron output such that: (i) the rise time for theneutron output to reach 90% of its plateau, measured from the time whenthe neutron output is 10% of the plateau, is less than 1 microsecond;(ii) the fall time between the instant when the voltage applied to thecathode is turned off and the instant time when the instantaneousneutron output falls to 10% of its plateau, is less than 0.5microsecond; and (iii) the neutron output reaches a plateau whichremains constant within a 10% range thereof, over a pulse time widthcomprised between 18 and 25 microsecond; and determining from thesignals a characteristic of the earth formation surrounding theborehole.